KVM: x86: Skip APICv update if APICv is disable at the module level

Bail from the APICv update paths _before_ taking apicv_update_lock if
APICv is disabled at the module level.  kvm_request_apicv_update() in
particular is invoked from multiple paths that can be reached without
APICv being enabled, e.g. svm_enable_irq_window(), and taking the
rw_sem for write when APICv is disabled may introduce unnecessary
contention and stalls.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20211208015236.1616697-25-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>